In recent years, with advance in digital technology, electronic devices such as mobile information devices and information consumer electronics have been developed to provide higher functionality. Therefore, there is increasing demands for nonvolatile memory elements incorporated in these devices to have a large capacity, reduced write power, high speed for write time and read time, and a long lifetime.
In response to such demands, miniaturization of existing flash memory using a floating gate is said to be limited. Thus, recently, there is increasing attention on novel variable resistance nonvolatile memory elements using a variable resistance layer as a memory unit.
The variable resistance nonvolatile memory element has a fairly simple structure in which the variable resistance layer is sandwiched between a bottom electrode and a top electrode. By only giving between the top and bottom electrodes a predetermined electrical pulse having a voltage equal to or greater than a certain threshold value, a resistance state of the variable resistance nonvolatile memory element changes between a high resistance state and a low resistance state. Then, information is recorded by associating these different resistance states and values with each other. Due to the simplicity in structure and operation, further miniaturization and cost reduction of the variable resistance nonvolatile memory element are expected. Furthermore, the resistance state of the variable resistance nonvolatile memory element may change between the high resistance and the low resistance on the order of 100 nanoseconds (ns) or less, and thus the variable resistance nonvolatile memory element draws attention from the stand point of high-speed operation, and there are various proposals.
Recently, there are proposals regarding variable resistance nonvolatile memory elements using materials, in particular, a metal oxide as the variable resistance layer. The variable resistance nonvolatile memory elements using such metal oxides are broadly categorized in two types, based on a material used as the variable resistance layer.
One type is a variable resistance nonvolatile memory element, disclosed in PTL 1 or the like, that uses a perovskite material (such as Pr(1-x)CaxMnO3 (PCMO), LaSrMnO3 (LSMO), and GdBaCoxOy (GBCO)) as a variable resistance layer.
The other type is a variable resistance nonvolatile memory element that uses a binary transition metal oxide which is a composition comprising solely a transition metal and oxygen. The binary transition metal oxide has a fairly simple compositional structure as compared to the above-described perovskite material. Thus, compositional control and deposition upon manufacturing are relatively easy. In addition, the binary transition metal oxide has an advantage that the binary transition metal oxide has relatively good conformity with semiconductor manufacturing processes, and thus, recently, extensive studies are made thereon.
For example, PTL 2 discloses a variable resistance element which uses, as a material of the variable resistance layer, an oxide having a stoichiometric composition or an oxide having a shortage of oxygen in the stoichiometric composition (hereinafter, referred to as an oxygen-deficient oxide), which comprises a transition metal, such as nickel (Ni), niobium (Nb), titanium (Ti), zirconium (Zr), hafnium (Hf), cobalt (Co), iron (Fe), copper (Cu), chromium (Cr).
Furthermore, PTL 3 discloses a nonvolatile memory element using an oxygen-deficient tantalum (Ta) oxide as a variable resistance material, and reports a resistance change phenomenon in a range satisfying 0.8≦x≦1.9 (from 44.4% to 65.5% in terms of oxygen concentration) when Ta-oxide layer is represented by TaOx.
Furthermore, PTL 4 proposes a variable resistance nonvolatile memory element having a stacked structure comprising Ta oxides having different oxygen concentrations.
Moreover, operationally, two different operation modes referred to as a unipolar (monopolar) switching and a bipolar switching are reported in the variable resistance nonvolatile memory element.
First, the unipolar switching is an operation mode in which electrical pulses having a same polarity of different magnitudes are applied between the bottom electrode and the top electrode to change a resistance value, which is disclosed in PTL 2 and the like. Moreover, as disclosed in PTL 4, in the unipolar switching, it is required to change the lengths of electrical pulses (pulse widths). For example, it is required to use two types of electrical pulses that have a length on the order of ns and a length on the order of microsecond (μs).
Meanwhile, the bipolar switching is an operation mode in which electrical pulses having different polarities which are positive and negative are applied between the bottom electrode and the top electrode to change the resistance value, which is disclosed in PTLs 1 and 3. As disclosed in PTL 3, electrical pulses for use in bipolar switching nonvolatile memory element are in general set to have a same length, and mostly on the order of ns. That is, a nonvolatile memory element allowing the bipolar switching has a feature that widths of the negative and positive pulses can be set to be fairly short and have a same length.
As described above, various types of variable resistance nonvolatile memory elements are proposed up to present. However, these variable resistance nonvolatile memory elements have a feature in common in which a predetermined voltage is applied to change the resistance state between the high resistance state and the low resistance state, and information is recorded by associating these resistance states and values with each other. Typically, it is defined that a state in which the nonvolatile memory element has a resistance value equal to or greater than a certain threshold value is the high resistance state, and a state in which the nonvolatile memory element has the resistance value less than the threshold value is the low resistance state. Moreover, typically, for example, data “1” and data “0” are assigned to the high resistance state and the low resistance state, respectively, in recording the information.
However, even if an attempt is made to apply a predetermined voltage to the element in the low resistance state to set the element to be in the high resistance state, the resistance value may not exceed the threshold value, ending up being a somewhat decreased resistance value. In contrast, even if an attempt is made to apply a predetermined voltage to the element in the high resistance state to change the resistance state to the low resistance state, the resistance value may not fall being equal to or less than the threshold value, ending up being a half-reduced value. In such cases, the element in the high resistance state may inadvertently be determined to be in the low resistance state, or the element in the low resistance state may inadvertently be determined to be in the high resistance state. That is, not setting a desired value to the resistance value leads directly to error in setting a memorized data.
To prevent such an error, for example, PTL 5 proposes a confirmation operation to verify whether the resistance value in the set resistance state satisfies for the threshold value. According to this method, for example, if it is desired to set the resistance value satisfying the high resistance state, a voltage which changes the resistance state to the high resistance state is applied to the element, and then the resistance value is read to determine whether the resistance value exceeds the threshold value. If the resistance value exceeds the threshold value, the setting for the resistance value ends. In contrast, if the resistance value does not exceed the threshold value, the voltage is re-applied to the element to re-set the resistance value. The resistance value is then re-read to determine whether the resistance value exceeds the threshold value. By repeating such operations, the resistance state of the element is set to be a desired state.